1. Field of the Invention
This invention relates to a relief valve for an oil pump, and more particularly to a relief valve for regulating discharged oil pressure of an oil pump.
2. Description of Related Arts
A conventional relief valve for an oil pump is hereinafter described referring to FIGS. 6 and 7. The relief valve for the oil pump (not shown) comprises a valve cylinder 231 including a valve bore 232 therein, a valve body 240 disposed slidably movable in the valve bore 232 in response to discharged oil pressure supplied by the oil pump, an inlet bore 233 for communicating between the valve bore 232 and a discharged port (not shown) of the oil pump formed opposite to the one side of the valve body 240, a spring (not shown) provided at the other side of the valve body 240 (right side in FIG. 6) so as to always urge the valve body 240 toward the inlet bore 233 (left side in FIG. 6) and a pair of relief grooves 234 having a respective openings formed in the inner peripheral of the valve bore 232.
When the discharged oil pressure of the oil pump is higher than the predetermined pressure level, the valve body 240 receiving the discharged oil pressure, slides against an urging force of the spring in the valve bore 232. So the opening area of the pair of relief grooves becomes large. When the discharged oil pressure is lower than the predetermined pressure level, the valve body 240 is not able to slide against the urging force of the spring. So the opening area of the pair of relief grooves is closed by the valve body 240. Namely, the valve body 240 regulates the discharged oil pressure to be maintained at the predetermined pressure level by opening and closing of the pair of relief grooves 234 relative to the valve body 240.
According to the above relief valve, the respective opening of the pair of relief grooves is formed perpendicular to an axial center line of the valve bore 232 and extending toward the other side of the valve body 240. When the discharged oil pressure exceeds the predetermined pressure level, a change rate of the opening area of the pair of relief grooves 234 relative to a slide movement of the valve body against the force of the spring becomes large rapidly. This causes a pressure fluctuation of the discharged oil by opening and closing of the relief bores 234, which leads to a vibration of the valve body 240, an unusual sound or an eccentric wearing on the valve body 240, and an invasion of foreign materials between the relief bore 234 and the valve body 240.
Another conventional relief valve is disclosed in a Japan Patent Laid-open Publication H05-332116 (JP 05332116 A). This relief valve is able to reduce the pressure fluctuation of the discharged oil because the opening of the relief bore is formed along an inner peripheral surface of a valve bore, having a convex portion toward one side of a valve body.
According to the relief valve, the discharged oil pressure in the valve bore is applied concentrically to the single relief bore, when the single relief bore is opened to the inner peripheral surface of the valve bore. This may cause the discharged oil pressure imbalance within the valve bore, which leads to an eccentric wearing on the valve body by being forced to move into the single relief bore.
Accordingly, it is an object of the present invention to provide an improved relief valve for an oil pump. It is another object of the present invention to provide an improved relief valve for an oil pump that obviates the above conventional drawbacks. It is a further object of the present invention to provide a simple relief valve for an oil pump to prevent a valve body from vibration and an eccentric wearing.
To achieve the above objects, relief valve for regulating discharged oil pressure of an oil pump at a predetermined level according to the invention, comprises a cylinder including a bore therein and relief grooves formed symmetrically relatively to an axial center of the bore, while each of the relief grooves has a respective opening to an inner peripheral portion of the bore, a valve body disposed in the cylinder so as to be slidably moved in the bore in response to the discharged oil pressure supplied into the bore at one side of the valve body, a spring provided in the other side of the valve body to always urge the valve body toward the one side of the valve body so as to close the openings of the relief grooves by the valve body, and adjusting means for adjusting a change rate of an opening area of the relief grooves relatively to the slide movement of the valve body against the urging force of the spring so that the change rate at the beginning of slide movement of the valve body is smaller than that at the end of the slide movement of the valve body.
According to the above relief valve, the change rate of the opening area of the relief grooves opened to the bore relatively to the slide movement of the valve body becomes smaller at the beginning than that at the end of the slide movement.
Accordingly, the adjusting means prevents changing the opening area of the relief grooves rapidly. Therefore it is possible to prevent vibration of the valve body, generating undesired sound, and an eccentric wearing due to the invasion of foreign materials between the pair of relief grooves and the valve body. Since the pressure balance within the bore can be kept at the opening operation due to the symmetrically provided relief bores, the pressure is equally applied to the valve body, which can prevent the eccentric wearing thereof.